During last few years, it has been reported great floods in Finland and
elsewhere in the world. For example in 2012 autumn, heavy and prolonged rains
caused flooding in Finland, and in 2013 there were widespread floods in Europe
due to the rainy spring. In addition to the prolonged rain periods, flooding
can be caused in Finland also by spring snow melt, frazil ice and ice dams, but
also they may relate to exceptionally high sea water levels. Presently, Finland
can be divided broadly into three main watershed areas. The coastal watersheds
are rather small, and they experience rain and snow melt induced flooding. The wide
lake area watersheds experience more long lasting floods and Northern rivers
experience mainly spring floods due to the snow melt.
According to the Intergovernmental Panel on Climate Change (IPCC) the
climate change may also effect on the hydrological cycle, and thus also on
flooding. The climatic change has been forecast to be greater in Northern
latitudes than the global average. Especially the extreme events have been
forecast to intensify.
Lately, there has been increasing amounts of studies related to the
climate change and its effects on flooding on global, regional and national
scale. In my PhD entitled “Fluvial processes and their future
magnitudes: combined field observation and simulation approaches”, it was shown
that due to the climatic warming, the discharges may change greatly in Finland.
However, the many hydrological scenarios had great temporal and spatial
differences between river reaches. Particularly great seasonal changes are to
be expected, since discharges may increase in autumns and winters. Also the
importance of the spring flooding as the channel modifying event is expected to
diminish from the present.
The great discharge events, for example the once having a 100 year
recurrence interval, are forecast to diminish on average by the end of the 21st
century. Particularly diminishing flood events are expected in Northern
Finland. On the other hand, the great discharges are expected to increase in
the lake area and their outlet rivers. One of these areas is Kokemäenjoki River
at Pori city area, which is already presently one of the greatest flood risk
areas in Finland. However, on average, the spring discharge peaks are forecast
to diminish. Important is that the autumn and winter discharges may increase in
Northern Finland, in the coastal area, and in the lake area. In addition, the
spring snow-melt discharge peak is forecast to occur two-three weeks earlier in
the end of the 21st century than presently. Even though the flood
inundation extent can be forecast to have same direction in the change as the
discharges, the change in flooding extent is not linear to the discharge
changes due to the local, particularly topographical, effects. In addition, the
sea level and its changes may locally have greater effect on flood inundation
than the changes in discharges.
According to the EU’s Floods Directive (2007/60/EC), also the climate
change impacts are to be taken into account in flood risk assessments. The 21 greatest
flood risk areas have been defined for Finland already in the end of 2011 by The
Ministry of Agriculture and Forestry. The city of Pori has been defined as the
greatest flood hazard and risk site, since there is a great amount of
population living in the flood risk area. The flood hazard and risk maps will
be created from these areas by the end of 2013 and the flood risk management
plans are to be made by the end of 2015. However, despite the already great
amount of studies related to flooding, further research on these issues is still
needed on global, regional and local scale along with continuously enhancing
future hydrological and flood simulation approaches.
References:
Lotsari, E. 2012. Fluvial processes and
their future magnitudes: combined field observation and simulation approaches. Doctoral
thesis (article-based). Department of Geography and Geology; Geography
Division. Annales Universitatis Turkuensis A II 270.
Ministry of the Environment. http://www.ymparisto.fi/default.asp?contentid=1712&lan=fi
Veijalainen,
N., Lotsari, E., Alho, P., Vehviläinen, B. and Käyhkö, J. 2010: National scale
assessment of climate change impacts on flooding in Finland. Journal of
Hydrology 391: 333–350.
